The present invention relates to electromagnetic print hammer actuators of the type described in the Canadian Pat. No. 1135317 particularly with reference to FIGS. 1a, 1b, 2, 3, 4 and 6.
Hammer actuators in the Canadian Patent comprise a stator and a movable armature member or ram that forms the hammer. The stator consists of two substantially symmetrically constructed magnetizable yoke halves with one or more excitation coils. Each yoke half is formed with two or more pole pieces. The yoke halves are positioned so that the ends of the pole pieces of one yoke half are aligned with the ends of the pole pieces of the other yoke half. The facing pole piece ends are spaced apart so as to form magnetic operation gaps. These gaps are aligned along the length of the stator.
The armature member or ram is located in the gaps between the stator halves and is movable relative to the stator in the direction of alignment of the gaps. The cross section of the ram is substantially the same as the cross section of the gaps defined by the shapes of the ends of the pole pieces. This cross section can be circular or rectangular. The body of the ram is made from a non-magnetizable material and includes armature elements of a magnetizable material. The armature elements extend substantially across the full width of the ram so that their cross section is substantially the same as the cross section of the ram and of the gaps. When the ram is in its rest position each armature element is located adjacent to a respective one of the magnetic operating gaps and the length of each armature element is substantially equal to the length of the associated gap. As a result the armature elements are geometrically shaped so that the volume of each armature element is of the order of the volume of the associated gap.
When the ram is in its rest position each armature element is located substantially outside its associated operating gap. Upon excitation of the coil or coils of the stator the resultant magnetic field causes each armature element to be pulled into its associated gap and the ram is accelerated along its length as it is constrained to move in a straight line. A hammer is formed at one end of the ram and this is used to perform a printing operation.
U.S. Pat. No. 4,425,845, describes an arrangement for accommodating a plurality of ram units positioned adjacent to one another into a hammer bank. Each ram unit consists of a flat, narrow frame. A tongue-shaped ram, to be driven by an electromagnetic actuator, is slidably received in a recess in the frame. For accommodating and guiding the individual print ram units, the bank is provided with a receiving bar and a comblike holding bar between which the frames of the units are arranged. Extension pieces of the lower elements of the frames are accommodated in slits in the receiving bar. The upper elements of the frames are held in recesses in the comblike holding bar. Each tongue-shaped ram is guided laterally by two electromagnetic actuators which are arranged one on each side of the frame and are aligned relative to each other.
Such a print hammer bank has a number of disadvantages including that it is expensive to manufacture and its ram units are difficult to exchange and are susceptible to oscillations.
There is described in IBM Technical Disclosure Bulletin, Volume 25, No 11B, April 1983, pages 6284/85 another type of print hammer bank in which a plurality of print hammer actuators are positioned adjacent to one another. Each actuator includes a stator having two yoke halves and a movable tongue shaped ram armature member formed at one end with a hammer. The rams are located in a common horizontal plane and for each ram the two stator yoke halves are located on opposite sides of this common plane. The rams extend closely adjacent to one another. The stator for each ram is wider than the associated ram and therefore extends beyond the sides of the ram. In order to reduce the overall length of the hammer bank the stators of adjacent rams are staggered. The stators for one set of rams are positioned towards the hammer ends of the rams and the stators of another set of rams which are interleaved between the rams of the first set are positioned towards the other ends of the rams.
The object of the present invention is to provide an improved print hammer bank which is compact and of such a construction that it is easy to replace defective components.
It is another object of the invention to provide a print hammer bank having a modular design which tends to eliminate the above mentioned disadvantages of known types of print hammer bank.